This invention relates generally to transmission systems and particularly to CATV transmission systems that have encoded video signals and that also transmit digital audio information. In a broader aspect, the invention envisions a signal formatting method having decided benefits and the appropriate transmitting and receiving apparatus therefor.
In CATV systems, scrambling or encoding of the transmitted television (TV) signal is common to prevent unauthorized use of the signals. Signal scrambling may take many forms, including random video inversion and deletion of synchronizing signals. The means for scrambling and recovering (or decoding) scrambled TV signals are well known in the art.
One popular form of encoding involves suppressing or deleting the information in the horizontal interval of the TV signal. That information consists of the horizontal sync pulse, and in a color signal, the reference color burst. The TV signal may be subjected to further scrambling by video inversion to assist a CATV operator in maintaining the security of his signal transmission. The advent of high quality stereo television broadcasting and third channel audio for foreign language transmissions, commonly referred to as SAP, has also given rise to a need to supply such information to CATV subscribers, preferably on a premium basis.
So-called digital audio has recently emerged as a means for providing high quality audio transmission because of its inherent immunity to noise. A digital audio signal is formed by dividing an analog audio signal into a fixed number of discrete amplitude levels, sampling the analog signal and generating a binary representation of the various levels of the sampled signal. Because of the nature of binary transmission and because of the discrete amplitude levels, any noise or deterioration experienced by the signals in transmission is readily eliminated. There are also techniques for compressing digital audio signals to facilitate data transfer in a limited bandwidth without sacrificing audio quality.
As in any digital data transmission system, reliable data recovery requires the recovery of a reliable data clock. To recover the clock, it is common practice to use either a self-clocking code or a clock run-in. These methods utilize time multiplexing of data and clock and thus reduce the available data rate. In its broad aspect, the invention enables efficient transmission of both the digital data and a clock reference which can be used to derive the data clock. This is accomplished by summing the two signals in an exact phase relationship such that separation of the data and clock reference is not necessary at the receiver before data slicing. Because of the use of frequency multiplexing, the clock reference can be recovered at the receiver with a phase-locked loop (PLL). In particular, the continuous wave (CW) clock reference is added to the digital data such that the zero crossings of the clock reference coincide with the data sampling points of the digital data. Since the CW clock reference signal has no amplitude at the sampling points of the data, separation of the CW clock reference is not necessary at the receiver before data slicing. The presence of the CW clock reference does not affect "eye height". In addition, the digital data may be pre-coded to redistribute the data energy away from the clock reference frequency to further improve the performance of the system.
In a specific aspect of the invention, efficient transmission of digital audio in a TV signal scrambling system is enabled by replacement of the information in the horizontal sync interval with pre-coded digital audio information and a clock reference, which is also used as a color burst for color television transmissions. The digital audio data is compressed to reduce its bit rate, pre-coded using either modified duobinary or duobinary and sent with a data rate of 2 Fc. A clock reference of Fc isused, where Fc is equal to the standard NTSC color burst frequency of 3.58 MHz. The pre-coded data and the color burst are inserted into the horizontal interval of a television signal on a pedestal of 50 IRE units. For duobinary pre-coding, a blank time interval is left in the horizontal interval during which no data is sent to provide a fixed 50 IRE level for automatic gain control (AGC) circuits in the receiver to use as a reference.
When modified duobinary pre-coding is used, two extra bits are added at the end of the data burst for DC balance. The nature of modified duobinary pre-coded data is such that the DC average of a burst of pre-coded data may always be made zero with the addition of two bits. This obviates the need for a blank time interval because the receiver AGC circuits may use the average level of the horizontal interval as its fixed reference. Thus the entire horizontal interval may be occupied by the pre-coded data and clock reference except for short front and back porch periods that separate the data from the video information. The two DC balance bits at the end of the data burst contain no digital audio information and are ignored by the audio decoder in the television receiver.